USMLE Step 1 Physiology Review 53 04 Neurotransmission

USMLE Step 1 Physiology Review 53 04 Neurotransmission

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Play USMLE Audio MP3 53 04 Neurotransmission Below

Begin 53 04 Neurotransmission Transcription

Now for some questions about neurotransmission.

Do propagated action potentials transfer electrical impulses directly?

  •  No.

How is neuronal communication generally accomplished?

  • It is accomplished via chemical transmission at synapses.

The next several questions deal with the presynaptic events; in other words, the excitation secretion coupling.

What is the immediate effect on the nerve terminal of the arrival of the action potential?

  •  Depolarization of the nerve terminal.

Due to this depolarization, what event occurs that involves calcium?

  • The entry of calcium into the nerve cell through voltage gated calcium channels.

The calcium now abundant in the presynaptic nerve terminal triggers another event, what is it?

  • The synaptic vesicles fuse with the presynaptic membrane.

What is contained by the synaptic vesicles?

  • Neurotransmitter.

What happens to the neurotransmitter when the synaptic vesicles fuse with the presynaptic membranes?

  • The neurotransmitter is released into the synaptic cleft by exocytosis.

What is the synaptic cleft?

  •  It is the space between the presynaptic membrane and the postsynaptic membrane.

What happens to the neurotransmitter after its release into the synaptic cleft by exocytosis?

  • It diffuses across the synaptic cleft.

Student Doctor, please pause the tape and summarize the presynaptic events.

  •  First, the action potential arrives at the presynaptic nerve terminal which triggers a depolarization of the nerve terminal.  Calcium enters the nerve cell through voltage gated calcium channels.  Then there is a calcium induced fusion of the synaptic vesicles and the presynaptic nerve membrane.  The neurotransmitter previously contained by the vesicles is released into the synaptic cleft by exocytosis, and the neurotransmitter diffuses across the synaptic cleft.

Okay, let’s do some questions on the postsynaptic events.

What happens after the neurotransmitter diffuses across the synaptic cleft

  • The neurotransmitter binds with the receptors on the postsynaptic membrane.

What happens next?

  • The receptors are activated, which causes changes in ion conductances.

What do these changes in ion conductances cause?

  •  They can cause either depolarization or hyperpolarization of the postsynaptic membrane.

And what does depolarization or hyperpolarization of the membrane trigger?

  • The dissociation of the neurotransmitter from the receptor.

If you were a molecule of neurotransmitter that just had dissociated from the receptor, where would you be?

  • In the synaptic cleft.

What happens to the neurotransmitter in the synaptic cleft?

  • The neurotransmitter is removed.

There are two ways the neurotransmitter is removed from the synaptic cleft; what is the primary way?

  • The presynaptic neuron takes up the intact neurotransmitter molecule.

What is the other way the neurotransmitter is removed from the synaptic cleft?

  • Enzymes degrade the neurotransmitter.

What is the approximate time delay between the arrival of the action potential at the presynaptic nerve terminal and the generation of action potential at the postsynaptic cell?

  • Point five milliseconds (0.5 ms)

What are the two events that account for most of this point 5 (0.5 ms) millisecond time delay?

  • The entry of calcium into the nerve terminal and the calcium induced release of neurotransmitter.

Student Doctor, please pause the tape and summarize the information discussed about postsynaptic events.

  • Neurotransmitters diffuse across the synaptic cleft and bind with the receptors on the postsynaptic membrane.  This binding activates the receptors, which causes changes in ion conductances.  The changes in ion conductances result in either depolarization or hyperpolarization of the postsynaptic membrane which causes the neurotransmitters to dissociate from the receptors on the postsynaptic membrane. The presynaptic neuron takes up the neurotransmitters left in the synaptic cleft after dissociation or enzymes may degrade the neurotransmitters left in the cleft.  The time delay between the arrival of the action potential at the presynaptic nerve terminal and generation of action potential at the postsynaptic cell is about point five milliseconds (0.5 ms).  The entry of calcium into the nerve terminal and the calcium induced release of neurotransmitter accounts for most of this time delay.

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